Bombs were planted outside Dallas Police headquarters in an attack early on Saturday that may have involved as many as four gunmen firing automatic weapons, police said.

Dallas Police Chief David Brown told reporters that a motive for the attack was not yet known, but he also said there had been threats and attacks on police elsewhere in the country in the past few months.

One of the devices, a pipe bomb, exploded when a police robot attempted to move it. Another, which was under a police vehicle, was detonated by a bomb squad, according to the police department.

Brown said witnesses reported that up to four suspects were involved in the incident, which began around 12:30 a.m.

No officers have been injured, but one suspect who has spoken with police said he had been hurt, Brown said.

The police chief said police had responded to reports of automatic gunfire from what was described as an armoured van outside police headquarters.

The van then rammed a squad car and gunfire erupted. The van drove off as police returned fire and officers gave chase, Brown said. Witnesses told police that one suspect may have failed to enter the van before it sped off, according to Brown.

The van stopped in a fast food restaurant parking lot in the city of Hutchins, some 16 km south of Dallas, where there was another exchange of gunfire.

Police said they surrounded the van and managed to disable it with a high-powered rifle.

Brown said police negotiators had spoken with someone inside the van, who identified himself as James Boulware.

The police chief said they had not yet been able to confirm the identity of the man, but said that police had responded previously to three incidents of domestic violence involving a man with that name.

Brown told reporters at the early morning news conference that the suspect had said that police had taken his child and had accused him of being a terrorist. The police chief said that the man then threatened to "blow us up."

Several bags were found scattered around police headquarters, two of which had explosives inside, police said. Another suspicious package was found in a dumpster near a different police station in the city, according to police.

Nearby residents were evacuated, Brown said.

A video of the shooting on social media showed a police officer approaching a black van with a flashlight as two other police cruisers pulled up behind the vehicle. The officer abruptly turned and ran away and then a volley of gunshots could be heard.

In another video, the van could be seen ramming a police cruiser before gunfire rang out again and the vehicle drove off.

The Dallas Morning News reported that police in the city of Paris, some 160 km northeast of Dallas, had arrested a James Boulware in 2013 after he had obtained firearms, ammunition and body armor and threatened to attack his family, schools, and churches.

Reuters could not confirm that the two incidents involved the same individual.-Reuters